LIFE AND ENERGY 7 



drop is in a state of tension. The surface takes that form in which 

 its area is the smallest possible, that is a sphere (E., p. 168). 



The surface of an amoeba is, then, in a state of tension, and if 

 the tension were the same everywhere, it would be a spherical 

 drop, like the oil. It does, indeed, become of this shape when 

 stimulated by an electric shock (E., p. 165). We will suppose 

 that it is, at a particular moment, spherical, and that at one part or 

 another something happens, either inside the organism or in the 

 water outside it, which makes the tension less at this spot. It will 

 be clear that the greater tension on the remaining part, which 

 tension, of course, produces a pressure throughout the liquid 

 protoplasm, will result in a pushing out of that part of the surface 

 where the tension is less. Thus a pseudopodium is formed. 



Here we may note a further proof of the liquid nature of 

 protoplasm. If the drops of liquid, which are usually present in 

 an amceba, sometimes containing organisms taken as food, be 

 observed, they will be seen to be spherical, whatever the shape of 

 the material inside them (p., Fig. 3, p. 2). The surface of the 

 liquid is free to take the form required by its surface tension. A 

 drop of fluid imprisoned in a jelly may be of any shape whatever. 



But how is this surface tension to be explained ? 



What, however, do we mean by "explanation " in science? We 

 have already " explained " Brownian movement by the kinetic 

 theory of gases, and the formation of pseudopodia by surface 

 tension, and what, we have actually done is to show that these 

 complex things are special cases of properties possessed by very 

 much larger groups of existences, not necessarily living. When we 

 now proceed to "explain" surface tension itself, what we do is to 

 show that it is a consequence of the properties possessed by liquids 

 as such. A further step might be to refer these properties back to 

 those of the molecule itself. It will be clear, nevertheless, that 

 scientific explanation must stop sooner or later. Even suppose 

 that everything has been explained in terms of the movement of 

 electric charges, this movement itself still remains a mystery. But 

 science does not pretend to be able to go beyond what can be 

 investigated by the powers we possess. 



As physiologists, our task is to refer, as far as we can, all 

 phenomena of life to the laws of physics and chemistry. At present 

 we have to be content, in many cases, with a reference to more 

 general physiological laws, applying to a larger group of phenomena 

 than the particular ones under consideration, but themselves still 

 " unexplained." 



And now we may proceed with that task. 



